Exec schema change and test script updates

[chungers]

This PR introduces a minor schema change for the config JSON for starting up plugins. Specifically, the configuration and specification of which executor (how the plugin is started -- using os or some other mechanism like Docker) is now better associated with the properties required by the executor. The JSON also follows the convention of using Properties for arbitrary block of JSON config.

• Updated the code for parsing the new schema
• Updated the end to end tester (scripts/tutorial-test2) and its config JSON (scripts/tutorial-start-plugins.json) to reflect the schema change.
• Updated the tester script (scripts/tutorial-test2) to be the target in make ci and send kill signals to the process group (of plugins) started by the CLI utility infrakit plugin start. This ensures proper cleanup of the test. Note that we made this possible by setting the plugin processes to be in the same process group as the CLI utility (see the config JSON for entries with property SamePgID set to true).

[GordonTheTurtle]

Please sign your commits following these rules:
https://github.com/docker/docker/blob/master/CONTRIBUTING.md#sign-your-work
The easiest way to do this is to amend the last commit:

$ git clone -b "exec-schema" git@github.com:chungers/infrakit.git somewhere
$ cd somewhere
$ git rebase -i HEAD~842353722064
editor opens
change each 'pick' to 'edit'
save the file and quit
$ git commit --amend -s --no-edit
$ git rebase --continue # and repeat the amend for each commit
$ git push -f

Amending updates the existing PR. You DO NOT need to open a new one.

[codecov-io]

Current coverage is 65.02% (diff: 100%)

Merging #358 into master will increase coverage by 0.14%

@@             master       #358   diff @@
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  Files            44         44          
  Lines          2110       2110          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
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+ Hits           1369       1372     +3   
+ Misses          599        598     -1   
+ Partials        142        140     -2   

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